Natural gas is normally available from gas fields at a pressure up to hundreds of atmospheres. Such fields are often far from transformation sites; the wellheads may be located, for instance, on offshore platforms or in subsea facilities. For reducing gas transfer equipment and piping costs, it is normal practice to reduce the gas pressure immediately after its extraction. To this purpose lamination valves, also known as choke-valves, are normally used where the natural gas undergoes a constant-enthalpy pressure reduction process with high irreversibility rate, in which the potential energy associated with the pressure of the gas is dissipated by friction. Therefore, the expansion is associated with a net energy loss.
On the other hand, the extraction sites are usually located in remote places such as offshore platforms, far from the electric energy distribution networks. Electric generators, that are usually provided for electrically supplying on-site electric equipment, consume a part of the extracted gas and may involve relevant installation and maintenance costs. Therefore the need is felt of a cheaper energy source for feeding such equipment, advantageously by exploiting the energy released by the expansion of the gas.
Upstream of the lamination valves, a separation treatment is carried out to separate solid matter, such as sand, salt etc., as well as a treatment for separating liquids, such as oil and water. This treatment, which is complicated by the high pressure of the gas, takes place in a structurally simple equipment, and retains only the largest solid residues and most of the water contained in the gas, but cannot eliminate the finest particulate, and small liquid droplets, which can however flow through the expansion valve without damaging or blocking it, since the valve is made of a wear resistant material. Furthermore, the expansion is accompanied by a cooling effect whose extent depends upon the pressure fall and which can lead to the formation of ice and of solid compounds like hydrocarbon hydrates, a process that can be put under control by injecting specific additives into the gas. Such a gas treatment is not normally adapted to prevent damages that may occur when expanding a gas in such energy recovery devices as conventional turbines, owing to residues such as sand and small water droplets that are present in the gas, or to solid matter that can be formed during the expansion. The maintenance and reparation required as a consequence of such damages can affect in an industrially unacceptable way the gas extraction unit.
Various systems have been proposed for recovering energy from the expansion of a gas at a wellhead.
For example, U.S. Pat. No. 3,808,794 describes a method and an apparatus for using the pressure of oil and gas wells to operate a turbine, while balancing the pressure in the line downstream of the turbine. In particular, this is obtained by means of a turbine-bypass line that is equipped with a regulation valve operated by a turbine downstream pressure control means, in order to maintain such downstream pressure at a constant value even if fluctuations of the turbine discharge pressure take place.
U.S. Pat. No. 4,369,373 describes a method and an apparatus for generating electricity by a generator comprising a motor that is arranged at a wellhead and that is operated by a fluid produced by a well. By controlling the electric load connected to the generator, it is possible to control the force required for operating the motor and thereby the pressure reduction across the motor. The electricity produced by the generator may be used in a heating element arranged downstream of the motor, for heating the fluid and preventing it from freezing, or for heating a particular emulsion separator. In the case of wells that produce both gas and liquid, separate motors are provided, as well as a gas/liquid separator.
U.S. Pat. No. 5,117,908 describes an installation comprising a turbine that receives crude oil from a well and, at a discharge side of the turbine, a means for separating the crude oil into a non-gaseous component and a gaseous component; a means is also provided for transferring the two components to respective conveying devices that are coupled with the turbine such that they can be operated by the turbine, thereby exploiting the crude oil pressure reduction that occurs in the turbine; in particular the conveying means is fitted on a shaft in common with the turbine. In an exemplary embodiment, the common shaft is provided with a coupling to an auxiliary electric motor and/or to an electric generator that can be connected to a remote electric user.
US 2005/0179263 describes a system for generating electric power in a subsea installation, where a turbine is operatively connected to a flow line to be operated by a fluid that flows along such flow line, thus generating an electric power. At least one speed sensor is provided for measuring the speed of the turbine. This patent application specifically relates to control and safety measures, and to passive safety means of the subsea installation.
Such systems, even if they refer to conventional energy recovering expansion devices, such as turbines, do not solve the problem of a fine separation of solids and liquids that are present in the material extracted from the well, nor they solve the problem of preventing the formation of solid material during the expansion within the turbine.
U.S. Pat. No. 3,749,162 relates to a method and to an apparatus for carrying out operations of extraction from an oil underwater extraction site in a zone where a surface ice layer is likely to form, which can move with respect to the underwater extraction site. In an exemplary embodiment, the extraction site comprises a conventional expansion valve for reducing crude oil pressure, and a system for injecting a glycol in the crude oil upstream of the expansion valve, in order to prevent a formation of ice during the pressure reduction, which system may be associated with a device for heating the glycol itself.